This invention relates to fins for a heat exchanger which have been treated to be hydrophilic and corrosion resistant.
Heat exchangers of various types have been used in a wide range of applications including room air conditioners, car air conditioners and air conditioners incorporating space coolers and heaters, for example. These heat exchangers are made preponderantly of aluminum and aluminum alloys. They generally comprise a zigzagging copper tube for carrying a coolant, refrigerant or the like and a multiplicity of fins disposed substantially in parallel to one another around the tube.
To reduce the size and improve performance, the designs for heat exchangers of this class of late have employed increasing numbers of fins and, therefore, have had an ever increasing available area of contact between the incoming air and the fins. For the same reasons, the space separating the fins is being reduced to the greatest extent possible without increasing the resistance to air flow between the fins.
When the surface temperature of the fins and the coolant tube falls below the dew point while the cooler is in operation, dew adheres to the surfaces of the fins and coolant tube. The dew adhering to the fins collects into hemispheres or spheres, which may grow until they reach the adjacent fins. When the dew reaches to the adjacent fins in this fashion, it can continue to collect by capillary action, clogging the spaces between the fins. This phenomenon is called bridging.
When the dew induces this bridging phenomenon, the resistance offered by the fins to the passing current of air increases notably, the heat-exchange ratio consequently is lowered and the cooling capacity of the heat exchanger degraded. These fins, therefore, should possess a hydrophilic surface.
The methods proposed to date for imparting a hydrophilic surface to the fins include forming thereon a coating containing a surfactant such as polyoxyethylene nonylphenyl ether on the surfaces of the fins, coating the surfaces of the fins with colloidal silica or water glass, and subjecting the surfaces of the fins to a post boehmite-treatment, for example.
Another hydrophilic coating comprises a proteinaceous substance having a peptide bond, i.e., gelatin. Further enhancement of the fins affinity for water is obtained by using a hydrophilic coat prepared by mixing a water soluble coating material such as acrylic paint, with the proteinaceous substance.
Other methods for coating fins may involve a phosphate treated aluminum surface which is processed directly with an aqueous silicate coating and then dried.
A still further method is coating an aluminum fin with an organic resin film having corrosion resistance over which a hydrophilic coating consisting of silicates such as silica sol, silicic acid and water glass is formed
In addition to the problem of providing hydrophilicity for the fins, corrosion between the copper tubes which carry the cooling agent and the aluminum fins present a further problem.